SEISMIC PROPAGATION PATHS, REGIONAL TRAVELTIMES, AND CRUSTAL STRUCTURE IN THE WESTERN UNITED STATES

Abstract

The U. S. Geological Survey, with VELA UNIFORM support, has recorded a network of about two thousand long‐offset refraction seismograms to provide information on the detection, location, and identification of seismic events. The measured velocities of compressional waves in upper‐mantle rocks, [Formula: see text], varies from at least 7.75 to 8.25 km/sec in the western United States. There seems to be a systematic variation of the velocity of [Formula: see text] with geologic environment and also with crustal thickness. Where the crust is thick (>35 km), measured values of the velocity of [Formula: see text] tend to be high (>8km/sec), and, where the crust is thin (<35 km), measured values of the velocity of [Formula: see text] tend to be low (<8 km/sec). The velocity of [Formula: see text] is generally low in the high mountains of the far west and generally high in the stable continental interior. The close relation between rock density and seismic velocity implies that, on a regional scale, a large part of isostatic compensation is achieved by density variations in upper‐mantle rocks.